In the field of sheet-fed offset printing it is desirable to be able to further reprint and process freshly printed sheet as quickly as possible, while at the same time still allowing the printing inks to settle in and on the surface of the paper in a way such that the desired print gloss and the desired resolution can be achieved. Relevant in this context are on the one hand the physical ink drying process, which is connected with the actual absorption of the ink vehicles into an image receptive coating, e.g. by means of a gradual system of fine to coarse pores or a special system of very fine pores. On the other hand there is the so-called chemical drying of the ink, which is connected with solidification of the ink in the surface and on the surface of the ink receptive layer, which normally takes place due to an oxidative cross-linking (oxygen involved) of cross-linkable constituents of the inks. This chemical drying process can on the one hand also be assisted by IR-irradiation, it may however also be sped up by adding specific chemicals to the inks which catalytically support the cross-linking process. The more efficient the physical drying during the first moments after the application of the ink, the quicker and more efficient the latter chemical drying takes place.
Nowadays typically times until reprinting and converting are in the range of several hours (typical values until reprinting for standard print layout: about 1-2 h; typical values until converting for standard print layout: 12-14 h; matt papers are more critical than glossy papers in these respects), which is a severe disadvantage of the present ink and/or paper technology, since it slows down the complete printing processes and necessitates intermediate storage. Today shorter times are possible if for example electron beam curing or UV irradiation is used after the printing step, but for both applications special inks and special equipment is required involving high costs and additional difficulties in the printing process and afterwards.
An improvement in this respect is described in WO-A-2007/006794 as well as WO-A-2007/006796. In a preferred embodiment of these two disclosures, the highly advantageous quick ink setting properties and chemical drying properties for offset printing are achieved by using a specific amorphous silica pigment, namely silica gel with a high (nano) fine internal porosity.